The invention relates generally to antennas for receiving RF signals and more particularly to the tuning of a resonant cavity formed on a printed circuit board.
Some antennas formed on a circuit boards have a resonant cavity defined by a ground plane on one side of the circuit board, a formed piece of stripline referred to as a stub on the other side of the circuit board and an electrical connection between them. The shape and length of the stub determines the resonant frequency of the cavity. Generally, the stub is formed of stripline shaped on a circuit board. In order to tune these antennas, discrete components such as capacitors and inductors are used. For example, variable capacitors and variable inductors are used to tune the desired resonant frequency during the manufacturing process to compensate for manufacturing variability or substitutions of materials. However, variations in temperature such as that encountered by an automobile causes the characteristics of the discrete components to change, which in turn causes the resonant frequency of the antenna to drift.
It is desirable to retain some manufacturing flexibility in an antenna design. For instance, if certain materials of the circuit board are unavailable during the life cycle of the circuit board substitute materials may be used. This may cause the resonant frequency to shift. Consequently, it is desirable to compensate for any frequency shift to facilitate retaining component material flexibility.